This disclosure generally relates to the measurement and control of the extension (or retraction) of scissor linkage mechanisms incorporated in scissor linkage systems, such as scissor lift devices.
Scissor lift devices are commonly used to lift workers and equipment during construction, painting, maintenance, assembly and manufacturing operations, including aircraft assembly. Scissor lift devices typically include one or more sets or stacks of scissor linkages operated by an actuator, such as a hydraulic cylinder, on a motor-driven base, and a payload platform mounted on the upper ends of the scissor linkages. The payload platform can be moved by extending or retracting the one or more sets or stacks of scissor linkages.
Scissor linkage mechanisms are commonly used in many types of applications, but measurement of the extension position and/or velocity of the platform (or an end effector mounted to the platform) is usually not available. One of the technical issues associated with scissor linkage mechanisms is that the motion of the payload platform has a non-linear relationship to the actuator position. This makes it difficult to measure the position and velocity of the platform or end effector and limits the usefulness of standard motion control techniques that rely on having a linear relationship between input and output.
For existing scissor lift devices, the operators do not know how high the lift has been extended, other than by visual estimation. The process to extend the scissor lift is performed by the operator watching visual landmarks as the mechanism is moving. Existing solutions typically use open-loop control where the operator holds a button which activates the actuator. The operator keeps the button pressed until the platform reaches the desired location, and then releases the button. With this form of human-in-the-loop control, operators have no way to automatically instruct the scissor lift to go to an exact location or return to a prior location. Also, since the extension speed of the lift is non-linear, the speed of the extension is not easy to control. In addition, since the position and velocity are not easy to measure and control in existing systems, automated control of the scissor lift devices has been limited to cases with simple on/off control, where physical limit switches are used to turn off the motion actuator.
Existing controllers for these types of devices usually rely only on a force input of the motion actuator, but since the rate of motion of the payload platform or end effector of a scissor lift is a non-linear function of the input, the motion rate changes as the scissor stack extends. This means that for a constant amount of input force, the extension velocity of the scissor lift will be changing throughout its motion range. This makes it difficult for the operator to provide constant velocity control, and makes it difficult (and possibly unsafe) to implement automated motion control.
Possible solutions to acquire the position and velocity of the platform (or end effector) of scissor lift devices could involve either direct physical measurement at run-time or table-lookup types of solutions.
For direct measurement, string encoders/potentiometers with very long strings attached between the base and the platform could be used if entanglement with the string is not a concern. But there can be problems with wrapping and stretch issues for long strings, resulting in inaccurate data. Another direct measurement solution is to use proximity sensors, such as laser-based distance measurement sensors, but these have occlusion issues.
Another common approach to addressing similar non-linear types of motion is to use a process based on table look-up. In these types of solutions the output variable (e.g., height) is measured at various known locations of the input actuator. This gives discrete output positions based on prior physical measurement. At run-time the system would use the input position to look-up the associated height in a table. Linear interpolation between stored points could be used to give approximation of height (with variable accuracy) between stored points, but if precise positioning is required, a new measurement will need to be taken at the desired position. Velocity control of the output would not be practical with this approach.
Providing continuous extension measurement for control of scissor linkage mechanisms would address applications requiring precise movement of the platform mounted to the scissor linkage mechanism, which would improve overall system performance. In addition, it would improve situational awareness, which may lead to improved safety of these systems. If position and velocity feedback were available to implement a continuous motion controller, then precise positioning could be achieved. With such capability, automated applications can be developed and enhanced collision avoidance and safety features can be implemented.